--is among the largest that the body uses on a regular basis; it is therefore very easy to interfere with its absorption (probably part of the reason B12 deficiency is more common than is generally believed).

Taking oral methylcobalamin (or even cyanocobalamin) im large doses is designed so that even through passive absorption, sufficient cobalamin will get through to provide enough for most normal needs. In a 1000mcg dose, perhaps 1-2% will get through (10-20mcg), but since figures on the daily requirement for cobalamin are usually set around 8mcg, this is usually enough for most people. People who are deficient or who need tissue repair often go above this though, often to 5000-10000mcg daily.

Still, it's probably best if one takes one's cobalamin seperately from anything else--I generally have mine at least two hours after a meal and at least one hour before, trying to time it to as empty a stomach as possible.